Hypercooling and thermodynamic properties of liquid Ni-Fe alloys

Ni-10% Fe and Ni-35% Fe alloy melts were undercooled with glass fluxing technique by 371 (0. 90 ΔT_h) and 343 K (0. 91 ΔT_h). Their hypercooling limits ΔT_h were determined to be 413 and 376 K respectively. A new method to measure the integral mean specific heat C_pL of undercooled metallic melts was developed according to the definition of hypercooling. That is, one can take C_pL as the ratio of latent heat of fusion ΔH_m to hypercooling limit ΔT_h, provided that the latter two physical parameters have been obtained through experiments. The C_pL values of the above Ni - Fe alloys were found to be 41. 0 and 40. 7 J/ (mol · K). Theoretical calculations were also carried out to reveal the relationships of enthalpy change ΔH_LS, entropy change ΔS_LS, Gibbs free energy change ΔG_LS and crystal nucleation rate I during rapid solidification vs undercooling. Although the maximum undercoolings achieved by both Ni - Fe alloys have exceeded the critical level of 0.2 T_L, the calculated results indicate that heterogeneous nucleation must have taken place in preference to homogeneous nucleation. Those heterogeneous nucleants with a wetting angle range of 53 ° < θ < 80 ° played the dominating role to prevent Ni - Fe alloys from further approaching their hypercooling limits.